模拟集成电路的低电压系统-Analog ICs for Lo

Introduction

Low voltages are most appropriate for use in systems where size, weight, and power consumption are especially critical such as PDAs, cell phones, and other battery-operated portable equipment. Low voltages also benefit line-powered systems because the lower power dissipation associated with lower voltage operation allows the use of smaller power supplies, heat-sinks, and fans.

This discussion covers many of the analog IC families mentioned above, the power savings inherent in their operation, and how to handle some of the issues associated with low-voltage operation. It also presents methods for generating the lower voltages from a 5V supply. Table 1 presents a selection of Maxim's 1.8V, 2.5V, 3.0V, and 3.3V integrated circuits.

Table 1. A Selection of Maxim's Low-Voltage Parts

Category

1.8V

2.5V

3V

3.3V

µP Supervisors

MAX6326

MAX6742

MAX6745

MAX6745

A/D Converters

MAX1122

MAX1462

MAX1115

MAX1144

Analog Switches

MAX4695

MAX4644

MAX4653

MAX4592

Charge-Pump Voltage Converters

—

MAX619

MAX1673

MAX1730

Inductor-Based, Step-Up Regulators

MAX1687

MAX1706

MAX1678

MAX1709

Inductor-Based, Step-Down Regulators

MAX1928

MAX1928

MAX1921

MAX1921

Comparators, Fast

LMX331

MAX997

MAX997

MAX997

Comparators, Low Current

MAX9020

MAX9020

MAX9020

MAX9020

Comparators, Low Offset

MAX966

MAX997

MAX997

MAX997

D/A Converters

MAX5522

MAX5523

MAX5811

MAX5355

Digital Pots

—

DS4301

MAX5408

MAX5408

Linear Regulators

MAX1589

MAX1818

MAX1589

MAX1818

Multiplexers

MAX4638

MAX4638

MAX4639

MAX4701

Op Amps, Fast

MAX4291

MAX410

MAX4489

MAX4489

Op Amps, Low Current

MAX4036

MAX4036

MAX4036

MAX4036

Op Amps, Low Offset

MAX480

MAX4236A

MAX4238

MAX4238

Real-Time Clocks

DS1375

DS1375

DS1375

DS1375

References

MAX6018A

MAX6029

MAX6029

MAX6029

RS-232

MAX218

MAX3381E

MAX3322E

MAX3232E

RS-485

—

MAX3471

MAX3471

MAX3076E

USB

MAX3453

MAX3453

MAX3453

MAX3453

Power Savings from Low-Voltage Operation

The power saved by reducing a system supply voltage from 5V to a lower value can be dramatic. For resistive and capacitive loads, power saved is proportional to the voltage squared. For constant-current loads such as references and op amps, the savings are linear. For constant-power loads such as hard-disk drives, the switch to low voltage does not save power, but requires the device to operate at a lower input voltage. Table 2 summarizes the power savings offered by lower voltage systems as compared to 5V systems.

Table 2. Power Savings Compared to 5V When Using Lower Voltages

Load Type

Power-SavingsFormula

Power Savings Compared to 5V (%)

1.8VOperation

2.5VOperation

3VOperation

3.3VOperation

Resistive

(1 - (V / 5)²) x 100

87

75

64

56

Capacitive

(1 - (V / 5)²) x 100

87

75

64

56

Constant Current

(V / 5) x 100

64

50

40

34

Voltage Regulation

General Information About Voltage Regulators

Maxim has scores of regulators that generate low voltage or convert low voltages to other levels. They include low-dropout linear regulators, switched-capacitor charge-pump converters, and inductor-based switching regulators.

Low dropout linear regulators (LDOs) are simple to use, require an input voltage greater than the output, and regulate up to 4A of output current. LDOs are less noisy and less expensive than switching regulators, but they dissipate more power (and generate more heat) as the applied input voltage rises. Linear regulators can, however, be quite efficient for applications with a low input-to-output voltage differential (efficiency is proportional to VOUT/VIN).

Charge-pump converters can boost a DC voltage to a higher value, or produce a voltage of opposite polarity from the input voltage. They use capacitors for energy storage and provide small, low-cost, DC-DC conversion circuits. However, they are limited in output current to less than 0.75A, and so they are restricted to low-power applications.

Inductor-based switching regulators offer both low- and high-power DC-DC voltage conversion, and can either step up (boost) or step down (buck) an input voltage. Maxim's boost regulators offer up to 4A of output current, and our buck regulators offer up to 60A of output current.

Low-Dropout Linear Regulators

Linear regulators require that the input voltage is greater than the output voltage. The minimum required difference between the input voltage and the output voltage is called the dropout voltage. Modern linear regulators have dropout voltages much lower than those of early linear regulators, like the LM7905, and have come to be called low-dropout regulators, or LDOs. Maxim offers a wide variety of LDOs with preset positive output voltages as low as 0.75V and adjustable voltages down to 0.5V. We also offer negative LDOs with outputs from -2.5V to -5V.

LDOs make regulating to a low DC voltage easy and economical. The MAX1589 LDO family is simple to use, feature preset output voltages that vary from 0.75V to 3.0V, and operate with less than 0.175V of dropout at their full 500mA of output current. The MAX1818 LDO family also offers preset output voltages from 0.75V to 3.0V, 500mA of output current, and feature adjustable output voltage down to 1.25V and a dropout voltage of only 0.12V at their full 500mA of output current.

Figure 1. The MAX1589 500mA LDO has preset outputs of 0.75V, 1.0V, 1.3V, 1.5V, 2.5V, and 3.0V. The MAX1818 500mA LDO has preset outputs of 1.5V, 1.8V, 2.0V, 2.5V, 3.0V, and 5.0V or an adjustable output from 1.25V to 5V.

The MAX8510 LDO series incorporates several features important in the design of portable equipment: small size (SC70), low dropout voltage (120mV at 120mA), and low supply current (90µA max.). The MAX8510/MAX8511 are preset to a variety of output voltages between 1.5V and 4.5V, while the MAX8512's output voltage can be adjusted with an external divider.

Charge-Pump Voltage Converters

For low-current applications, the simplest solution for boosting 3V to 5V is a capacitor-based regulating charge pump (Figure 2). The industry-standard 7660 and most other charge pumps don't regulate VOUT, but the MAX619 includes an analog reference and error amplifier whose output controls a set of internal switches connected to external capacitors. The switch-capacitor network can double or triple VIN, and the MAX619 regulates by switching between these modes of operation. As indicated, this circuit produces 20mA at 5V ±4% for inputs that range between 2V and 3.6V. For inputs between 3.0V and 3.6V, the output-current capability is 50mA.

Figure 2. Occupying less than 0.1in² of board space, the MAX619 regulated charge-pump converter generates 20mA at 5V ±4% for inputs of 2V to 3.6V and 50mA from 3V to 3.6V.

The MAX5008 regulated 5V USB charge pump provides an output current of up to 125mA with supply voltages down to 2.9V, and is suitable for USB host devices with 3.0V or 3.3V supplies.

Systems that handle bipolar signals usually require a negative supply, which can be generated locally if necessary. A simple solution is to use a charge-pump inverter such as the MAX889.

Inductor-Based, Switch-Mode Regulators

Switching regulators provide single or multiple outputs. Depending on the output power level, these devices are controlled by pulse-frequency modulation (PFM), pulse-width modulation (PWM), or both.. PFM control schemes, which allow high efficiency with light loads, permit the regulator to operate with quiescent supply currents as low as 10µA. PWM schemes consume more power, but they allow fixed-frequency operation for better control of noise and EMI. Some converters switch between the two control schemes based on a control signal or the load-current level.

While LDOs are efficient for low values of (VIN - VOUT), many applications have an input voltage that is significantly higher than the output voltage. In this case, efficiency dictates the use of a buck-switching regulator. RF applications such as radios and cellular phones can easily use switching regulators, but care must be taken to avoid introducing noise at the sensitive IF frequencies.

An ideal choice for RF applications is the MAX1684 switching regulator. This device delivers 1A at 3.3V from inputs of 4V to 12V with efficiencies up to 97%, has a 300kHz internal oscillator, and features internal MOSFETs for ease of use.

An efficient buck regulator is also a good choice for upgrading an existing logic board to accommodate lower-voltage, lower-power ICs. These boards typically have 3.3V available, but they require a 1.8V supply for the new low-voltage logic. A linear regulator can easily convert 3.3V to 1.8V, but the power dissipation is prohibitive for higher load currents. At 2A, for instance, the linear regulator would dissipate 3W and require a heat-sink. The MAX1830 high-efficiency switcher (Figure 3), however, delivers 20mA to 2A with efficiencies exceeding 90% and requires no external MOSFETs, thus eliminating the need for a heat-sink.

Figure 3. The MAX1830 switching regulator converts 3.3V to 1.8V with efficiency greater than 90% from 20mA to 2A, and requires no external MOSFETs.

The MAX1830 comes in a very small, 16-pin QSOP package and has an input range of 3V to 5.5V. Its quiescent operating current drops from 325µA to only 0.2µA in standby mode. High switching frequency (up to 1MHz) allows the device to operate with small, low-cost, surface-mount components.

Multi-output switching regulators are intended for systems designed to operate with multiple supplies. Applications such as the generation of VCC voltages in a notebook computer, for example, are well served by the